Because osteoarthritis (OA) is a highly prevalent public health problem, even small improvements in health and well-being among people with OA (PWOA) can have a substantial impact. Physical activity is central to managing OA, but most PWO are insufficiently active so they do not benefit from it. This proposal targets insufficiently active PWOA, with the goal of developing a novel intervention approach to help them make small but sustained increases in lifestyle physical activity (LPA). Partner support (e.g., from a spouse) has the potential to be a particularly powerful resource for achieving this goal; social support is one of the most reliable psychosocial predictors of physical activity in PWOA and partners are ideally situated to assist with and encourage lifestyle change. Yet, evidence shows partner support can hinder instead of promote behavior change. Current understanding of partner support processes is insufficient to guide development of partner support or couples interventions that are reliably effective. Our social support effectiveness (SSE) framework can help elucidate these processes. It specifies features of partner support that maximize its potential to promote LPA among insufficiently active PWOA and suggests that these beneficial features of partner support are most likely to be present when both members of a couple have adequate support-related skills. Further, evidence shows that the most critical support-related skills involve emotion regulationability to understand one's own and another's emotions and to respond adaptively to discordance. We propose a 2- phase line of research. The Foundational Phase is a dyadic longitudinal study of 167 couples from the Johnston County OA Project. It will identify interrelations among partner support processes, PWOA and partner emotion regulation skills, and pathways through which these factors influence initiation and maintenance of LPA among insufficiently active PWOA over the course of a year (Aims 1-3). The Translational Phase is a series of three studies to evaluate potential to translate this new knowledge into a novel training program that improves couples' emotion regulation skills and their ability to apply them to receiving and providing effective partner support for LPA (Aim 4). This proposal benefits from a well established resource for recruiting our population, an expert team with established collaborations, and a strong theoretical basis. It targets a significant public health problem and promises to enhance the effectiveness of LPA interventions for insufficiently active PWO by leveraging partners as an effective resource to support their lifestyle change.